1. Field of the Invention
The present invention relates to a trim-tilt device for a marine propulsion unit such as an outboard motor or inboard/outboard motor.
2. Description of the Related Art
Conventionally, the trim-tilt device for a marine propulsion unit has a cylinder device interposed between a hull and the propulsion unit which is tiltably supported by the hull. By controlling supply and discharge of hydraulic fluid from a hydraulic fluid supply/discharge device to a cylinder device or vise versa, the cylinder device is expanded and contracted to thereby trim and tilt the marine propulsion unit.
A prior art trim-tilt device for a marine propulsion unit is described in Japanese Patent Application No. 11-112856. This application suggests a cylinder device for a trim-tilt device for a marine propulsion unit comprising a housing connected to one of a hull and a marine propulsion unit and which forms a large-diameter trim chamber. A cylinder is telescopically inserted into the trim chamber of the housing which forms a small-diameter tilt chamber. A large-diameter trim piston is fixed to an end portion of the cylinder within the trim chamber of the housing which partitions the trim chamber into a first trim chamber of a cylinder accommodation side and a second trim chamber of an anti cylinder accommodation side. A piston rod is connected to the other of the hull and the marine propulsion unit and is telescopically inserted into the tilt chamber of the cylinder. A small-diameter tilt piston is fixed to an end of the piston rod within the tilt chamber of the cylinder which partitions the tilt chamber into a first tilt chamber of a piston rod accommodation side and a second tilt chamber of an anti piston rod accommodation side. Additionally, in the cylinder device, a portion of the cylinder or the cylinder device which projects outward from the housing in the trim operable region is covered with a tank housing constituting a hydraulic fluid supply/discharge device. According to this embodiment, since the cylinder of the cylinder device does not touch water in the outside, it is covered with the tank housing and it can be rustproof easily and steadily by hydraulic fluid within the tank housing.
In the above mentioned trim-tilt device for a marine propulsion unit disclosed in Japanese Patent Application No. 11-112856, if the operation of the pump of the trim-tilt device stops in a trim operable region (the state in which the cylinder device does not finish the trim-up operation) during forwarding sailing and an obstacle in the water such as a floating log etc., collides with the propulsion unit such that a hydraulic fluid supply/discharge flow passage, from the hydraulic fluid supply/discharge device to the cylinder device, is locked, the piston rod and the cylinder in a body carry out a trim-stroke to the housing and the tank housing. At this time, in the trim-tilt device of the propulsion unit, due to the boosting of the hydraulic pressure at the locked state in a first tilt chamber, a buffer valve of a tilt piston is opened to transfer this hydraulic pressure to a second tilt chamber, so that an impact can be absorbed.
However, when the piston rod the cylinder transfer (trim-stroke) to the housing and the tank housing as described above, there is a possibility that the tank housing which is made of resin is destroyed due to the rapid increase of an internal pressure of the tank, or an end plate of the pump motor which is made of resin is destroyed, that covers a pump chamber provided so as to communicate to the tank.